The use of screen or filter devices for filtering extruded material such as plastics or rubber is well known. Filtering is particularly useful when extruding recycled plastics or rubber in order to remove particles of dirt or foreign matter from the recycled material. Filters utilized in such recycling operations are, however, particularly subject to clogging. Accordingly, the changing of screen devices or filter elements at frequent intervals has been necessary.
Mechanisms for changing screen devices associated with plastic extruders are well known. Examples of such devices can be seen with reference, for example, to U.S. Pat. Nos. 642,814; 1,195,576; 3,797,665; and 4,019,987. Typically, such mechanisms include a slide assembly which moves a clean filter screen contained in a support into the path of the extrudate while simultaneously moving the used, clogged filter screen to a standby station where the screen itself is replaced. Most screening or filtering media used for filtering extruded plastic materials are discarded and replaced with new screening material. Discarding of such screens, such as wire mesh screens, becomes more and more costly as the frequency interval between required screen changes becomes shorter. Furthermore, the replacement or change from a clogged screen to a clean screen should be done relatively rapidly or in a manner so that a clean screen can readily be provided in the extrusion path with no or minimal interruption of the extrusion process. In fact, automatic screen cleaning would be a desirable feature, not only to minimize the replacement time, but also to minimize any inconvenience associated with manual replacement. Generally, all of the known screen changers effect replacement of the dirty screen devices manually. Exceptions to this are the automatic screen changers described in my prior U.S. Pat. No. 4,082,487 and my application Ser. No. 972,633. In all instances, however, known methods and devices for providing clean screen surfaces to replace dirty clogged screen surfaces in extrusion apparatus operate by actually moving the screen or filter and replacing the screen or filter with a fresh one. It would be highly desirable to have a filtering device or mechanism associated with an extrusion apparatus wherein the screen or filter itself may be cleaned in situ while the extrusion operation proceeds and wherein the screen or filter element need not be disposed of, but rather, can be re-used after cleaning for extended periods of time. By further providing an automatic means for accomplishing the above, a substantial saving in both labor and screen materials may be achieved.